Cultural trauma theory has emphasised the role of social groups in narrating, and thereby attributing moral significance to, highly disruptive events. In contrast, this article draws attention to professions such as the police and the media, which act as “fact-finders” to establish the factual circumstances of events from which trauma narratives are created. The article offers a case study of the June 1994 Matsumoto Sarin Incident in Japan, a terrorist attack in which members of religious movement Aum Shinrikyō gassed residential streets using sarin. Factual uncertainties surrounding the attack, in combination with institutional failings by “fact-finders” that resulted in a false accusation, meant that carrier groups did not identify the event as one that brought a collectivity underlying values into question; in other words, cultural trauma as a discourse did not develop. It was only after Aum’s second sarin attack on the Tokyo subway in March 1995, when the true perpetrators and motives were finally uncovered, that the Matsumoto belatedly became recognised as a traumatic assault on Japan’s civic values. This article suggests that a collaborative approach combining science and technology studies (STS) with collective memory studies could provide a fruitful avenue of further research.